System, Apparatus, and Method for Repreoducible and Comparable Flow Acquisitions

ABSTRACT

A system, apparatus and method are provided for measuring and removing the influence of pulsatility on contrast agent flow in a region of interest of a vascular system of a patient. Once the change of blood speed over the cardiac cycle is known (pulsatility), this influence is removed from acquired image sequence for outcome control such that “quasi-stationary”, regular flow acquisition is passed on to subsequent visualization and analysis processes. A contrast agent injector is also provided that simultaneously measures and uses ECG to inject a known contrast agent at a fixed point over the cardiac cycle or such that a known amount of contrast agent will arrive at a known time at a region of interest in the vasculature of a patient, thus controlling one of the main unwanted variables in an acquisition of blood flow sequences.

The present invention relates to a system, apparatus, and method for acquiring information on blood flow that is both reproducible and comparable.

Blood is pumped through the body because pressure is imposed on the blood by the contraction of the heart. The instantaneous amount of pressure imposed on the blood depends on the contraction status of the heart, i.e., on the phase of the heart cycle. During systole the pressure is high, whereas the pressure might be nearly zero during diastole. Starting from the heart the time-varying pressure wave propagates through the vessel system leading to varying velocities v(t)=v_(average)p(t) in the vessel system. The instantaneous blood propagation speed in the human vascular system strongly depends on the contraction status of the heart, i.e., on the phase of the heart cycle. The phenomenon is called the pulsatility p(t) of blood flow. The degree of pulsatility, here defined as the strength of variation of flow speed over the cardiac cycle, is an important functional parameter that gives the physician fundamental information about the actual state of the vascular system which includes but is not limited to:

-   -   Degree of arteriosclerosis/vessel wall compliance: A high degree         of pulsatility shows that the vessel wall is not flexible any         more and indicates calcification; and     -   Risk factor for aneurysms: A high degree of pulsatility leads to         strong varying pressure waves affecting the weakened vessel         wall.         In addition, pulsatility p(t) influences and obscures functional         parameters that describe other aspects of blood flow and         perfusion. If the dependency of these other functional         parameters on pulsatility is describable, one can compensate for         the effects of pulsatility. As a result, the measurement of such         functional parameters becomes more reliable and reproducible,         e.g., allowing a better comparison of pre- and post-treatment         parameters. An example is the instantaneous volumetric blood         flow Q(t)=Q_(average)p(t).

Despite its clinical relevance no methods have yet been established that estimate the degree of pulsatility from routinely acquired angiographic image sequences. That is, there is no generally accepted way for a physician/interventionalist to use acquired data for reliable and reproducible estimation of pulsatility and/or degree of pulsatility mainly from contrast agent density variations resulting from injection of a contrast medium. In providing such a technique or model, the model describing the influences of pulsatility for the mixing of blood and contrast agent and the transport of this mix in the vascular system under the influence of pulsatility must be adapted to the acquired observations that show the advance of contrast agent through a vascular system such that model parameters characterize the pulsatility. As one possibility, a time-intensity-curve of the concentration of contrast agent extracted at or near the point of contrast agent injection can be used as a feature that is input to this analysis.

Blood flow visualization and measurements are essential for assessing the severity of diseases in arteries (e.g. stenoses or aneurysms). One important application is blood flow assessment that is based on the visualization of the intra-arterial contrast bolus in contrasted diagnostic acquisitions of vasculature. Examples are given for the case of interventional x-ray but not intended to limit the applicability of the described methods and apparatus to this clinical application. In a minimally-invasive procedure an interventional radiologist inserts a catheter into the vessels of interest and injects contrast agent to make the blood flow visible in a sequence of images. Subsequently, the physician assesses the blood flow by a visual inspection of the spreading of the contrast agent in the recorded image sequence. Due to the pulsatile nature of blood flow the instantaneous propagation speed of the bolus strongly depends on the phase of the heart cycle. During the systole the contrast agent will be quickly pushed forward, whereas the bolus might even come to rest during the diastole. Diminishing the influence of the heartbeat eases the evaluation of the relevant physiologic information. As a result, blood flow assessment becomes easier and more reliable for the physician. Additionally, the dependency on injection point and time is reduced making the blood flow assessment more reproducible and allowing a better comparison of pre- and post treatment angiograms for outcome control.

Experiments on clinical data show that prior art image analysis methods to quantify contrast agent (CA) dynamics do not always give reproducible results. However, without such reproducibility, even a comparison pre- and post-intervention as outcome control is not valid. As major cause for non-reproducibility, a dependency of the CA dynamics to heart pulsatility was detected. Especially for elderly patients with increasing degree of arteriosclerosis, the vessel wall compliance is reduced and, consequently, blood flow in arteries shows strong variation over a heart cycle respective to the varying driving pressure in the aorta.

Given this condition and an arbitrary start of a contrast agent injection, a videodensitometric assessment of blood flow dynamics depends on the phase in the cardiac cycle when the CA bolus front has passed a diagnostically relevant region of interest in a vascular substructure. In the aorta, an oscillating flow velocity v(t) 1202 in vessels with reduced compliance is estimated.

A technique is needed that provides the physician/interventionalist with a technique that is able to measure pulsatility without any changes in the workflow and in the setup, uses this measurement of pulsatility to correctly subtract out of images the dependency of bolus propagation on the instantaneous phase of the heartbeat, and that provides a means for timing CA injection such that the effect of a patient's heart beat is reduced. Further, there is a need for a technique that can be used for a comparison of acquisitions before and after an interventional step and to separate the influences of the total blood flow amount and its pulsatility over the cardiac cycle.

The present invention provides a system, apparatus, and method for acquiring information on blood flow that is both reproducible and comparable. In the acquisition of such blood flow, for the physician/interventionalist the degree of pulsatility is an important functional parameter that among other clinical information:

-   -   indicates the actual state of the vascular system, e.g. the         degree of arteriosclerosis or the vessel wall compliance;     -   provides one risk factor for aneurysms; and     -   influences and obscures other functional parameters like the         volumetric blood flow.         Hence, it is essential to determine functional parameters that         take into account pulsatility and make these functional         parameters more reliable and reproducible, e.g., allowing a         better comparison of pre- and post-treatment parameters.

A first embodiment of the present invention provides the physician with a system, apparatus and method that is able to measure pulsatility from angiographies without any changes in a workflow and setup. The present invention provides a measurement of pulsatility from angiographies that is easily applicable to clinical sequences and may be performed fully automatically. In addition, estimation of pulsatility is a prerequisite for a reliable and reproducible blood flow assessment.

In the first preferred embodiment, the pulsatility and/or the degree of pulsatility is estimated primarily from contrast agent density variations resulting from injection of the contrast medium. A model describing mixing of blood and contrast agent and the transport of contrast agent in the vascular system under the influence of pulsatility is adapted to a time-intensity-curve. The model comprises all relevant parameters for the characterization of pulsatility and incorporates, in particular, the influences of pulsatility on the mixing of blood and contrast agent and for the transportation, by the flowing blood in a patient, of contrast agent having varying velocities.

In an alternative first embodiment, additional information is taken into account including the duration of each heart cycle extracted from a simultaneously recorded ECG signal.

The basic idea of a second embodiment is to replay the acquired x-ray sequence according to a time axis transformation making the visualization and subsequent processing of this bolus propagation less dependent on the instantaneous phase of the heartbeat. The influence of pulsatility is removed from the acquired image sequence for outcome control such that “quasi-stationary”, regular flow acquisition is passed on to subsequent visualization and analysis processes. The characteristic parameters can be determined automatically (e.g. by image processing, preferably using the first embodiment described above or using ECG information) or selected by the end-user. For an optimal image quality of the replay and for a minimal dose exposure of the patient a time varying frame rate or a high frame rate with varying dose should be used for the acquisition of the angiograms. In doing so, the same time axis calibration as for the replay can be exploited.

In an extension of the proposed approach an improved outcome control is conducted by the comparison of adjusted pre and post treatment angiograms. To this end, both replay sequences have to be temporally adjusted to compensate for pulsatility such that the bolus starts from a given reference location (e.g. injection point) at the same time. Subsequently, the sequences are replayed to the physician in comparison or after respective subtraction if a visualization of the differences in blood flow after an intervention step is desired.

In the assessment of blood flow the physician has to take into account that the displayed propagation of the bolus is influenced by both physiological parameters indicating the degree of the disease (like blood flow obstructed by a stenosis, i.e. a local reduction of the cross section) and the phase of the heartbeat. The superimpositions of both as well as the different time scales due to the pulsatile nature of blood flow complicate assessment of flow sequences and diagnosis. Hence, the second embodiment provides an improved display of flow sequences to the physician by reducing the dependency of the bolus propagation from the instantaneous phase of the heart cycle. Since the bolus propagation is then also less dependent on the injection point and time, the procedure becomes more reproducible and allows a better outcome control by the comparison of pre- and post treatment angiograms.

Since there is a growing demand for functional information, the functionality of the second embodiment and its alternatives are important extensions of the application range of the first embodiment. The new time axis transformation can easily be added to planned perfusion and flow software packages and increases the value of angiograms for diagnoses and outcome control. One of the most important applications of the proposed method is visualizing differences in blood flow before and after an intervention. Note that this comparison is impossible if the proposed sequences are simply subtracted from one another, in which instance the sequence would be massively obscured by the fact that slow and fast flow time spans intermingle with different frequencies because of the effect of the patient's heart beat.

In a third embodiment, the control of the CA injection is based on an ECG analysis. When the physician/interventionalist initiates the contrast agent injection, the injector does not start immediately, but rather starts automatically at a next appearance of a pre-defined cardiac phase in a live-observed ECG.

This third embodiment can be used for achieving:

-   -   outcome control based on comparison of CA dynamics pre- and post         intervention which is possible if a reproducible CA injection is         provided.     -   different injector patterns can be adapted to the current heart         rate and assessment algorithms can request the phase in the         cardiac cycle when an observed bolus front arrives at a region         of interest in a vascular subsystem.     -   the automated control of short contrast agent bursts, such that         a flow dynamic at a pre-selected phase in the cardiac cycle can         be assessed.

FIG. 1: Comparison of injection pattern (black) and time intensity curve at injection point (pulsatile initial injection, grey);

FIG. 2: Setup for the pulsatility measurement;

FIG. 3: Calculation of the time intensity curve I₁(t) at an observation point downstream of the injection site;

FIG. 4: Example for a measured time intensity curve and the corresponding model fit of the preferred analysis method;

FIG. 5: Modified workflow for the enhanced displaying of x-ray flow sequences (TAT=time axis transformation);

FIGS. 6 a-b illustrate a comparison of pre- and post-intervention images using subtraction provided by the second embodiment;

FIG. 7 illustrates an apparatus for extracting pulsatility from an acquired sequence of images according to the present invention;

FIG. 8 illustrates an apparatus 800 according to a third embodiment; and

FIG. 9 illustrates an analysis system embodiment.

It is to be understood by persons of ordinary skill in the art that the following descriptions are provided for purposes of illustration and not for limitation. An artisan understands that there are many variations that lie within the spirit of the invention and the scope of the appended claims. Unnecessary detail of known functions and structure may be omitted from the current descriptions so as not to obscure the present invention. This is especially true of implementation details, even though some such implementation details are included in a drawing for an injection pump, this is a preferred mechanism. However, such a pump is not the only control mechanism and is used herein for expository purposes only.

In order to make blood flow visible in a sequence of images, the physician/interventionalist injects contrast agent into the vessels of interest which is subsequently imaged with an acquisition modality with sufficient temporal resolution. Such modalities include but are not limited to x-ray angiography, ultrasound, computed tomography and magnetic resonance imaging. In such acquisitions, the appearance of contrast agent in the image sequence is influenced by different pulsatility mechanisms:

-   -   A. Transformation of original injection pattern at injection         point: At the injection point, contrast agent and blood is         mixed. According to the ratio of these components, the vessel         appears on the x-ray image with certain intensity I₀(t).         However, the ratio of components changes subject to the         instantaneous volumetric blood flow

Q _(feed)(t)=Q _(average) p(t).  (1)

-   -    Hence, if this quantity is high, the fraction of blood is         higher and therefore the attenuation of the vessel on the x-ray         is lower. When the instantaneous volumetric blood flow is low         the fraction of blood is also lower and the attenuation of the         vessel is higher (FIG. 1).     -   B. Transporting contrast agent with varying velocities: Since         the instantaneous velocity depends on the current phase of the         heart cycle and the distance to the centerline of the vessel,         contrast agent is transported with varying velocities. As a         result, the appearance of contrast agent is not only modified on         its way from the injection point to the observation point by         effects like diffusion, but also due to pulsatility.

A method for a preferred first embodiment of the invention comprises the following steps:

-   -   1. Injecting contrast agent: via a catheter into the vessel tree         of interest. The injection point may not be visible on the x-ray         image and the distance L between injection and a suitably chosen         observation point can be an additional model parameter (FIG. 2).     -   2. Recording of ECG: is not necessarily required. However, by         extracting the duration of each heart cycle from the ECG signal         the number of parameters of the pulsatility model is reduced.     -   3. Extracting time intensity curve I₁(t) at the observation         point which determines the concentration of contrast agent at         this point over time: by averaging the intensity in a certain         area around the observation point in a vessel in order to reduce         the influence of noise.     -   4. Setting up of an appropriate model for the transport of         contrast agent under the influence of pulsatility p(t): A         suitable model has to comprise the different influences of         pulsatility like the         -   Pulsatile initial injection and         -   Pulsatile contrast agent transfer (CAT).     -    Via the resulting model, a prediction of the time intensity         curve at the observation point is possible. An exemplarily         pulsatility model is described in the following:         -   a) The pulsatile injection I₀(t), which is the time             intensity curve at the injection point (FIG. 2), depends on             the ratio of contrast agent (CA) and the overall amount of             fluid (blood and CA). Due to pulsatility the amount of blood             Q_(feed)(t)

$\begin{matrix} {{I_{0}(t)} = \frac{c_{CA}{Q_{CA}(t)}}{{Q_{CA}(t)} + {{\overset{\_}{Q}}_{feed}{p(t)}}}} & (2) \end{matrix}$

-   -   -   according to Eq. 1 at the injection point is time dependent,             whereas the amount Q_(CA)(t) of CA is only time varying if             the injection pattern is not a rectangular impulse.             Templates for the pulsatility p(Ø_(ECG)) in dependency of             the phase Ø_(ECG) in the cardiac cycle are readily available             for all observable vascular subsystems in the human body.             For each observation, these templates have to be scaled and             shifted in p and Ø-direction to reflect changes in the total             blood flow, the degree of pulsatility, and a temporal offset             from the ECG signal to the observed curve. From the             observation itself or additionally acquired ECG, the phase             in the cardiac cycle can be determined as Ø_(ECG)(t) for             each time of observation to construct the target p(t).             Parameters like c_(CA) further describe the influence of             contrast agent on image values (if the system is not             calibrated beforehand), Q_(CA) (if the injection pump does             not give the volumetric contrast agent flow) and Q_(Average)             are additional parameters in the optimization routine that             are not directly related to pulsatility. When the contrast             is injected through long, thin, flexible catheters, then the             contrast agent at the injection site follows the contrast             agent injector with a dampening and delay that is             represented by a T1 control element. Here, the strength of             this effect can be determined beforehand when the catheter             is known.         -   b) After the pulsatile injection I₀(t) the CA is subject to             a pulsatile contrast agent transfer. The intensity I₁(t_(o))             at the observation point at any observation time t_(o) is             obtained by integrating the intensity values I(r,t_(o)) over             the vessel lumen (FIG. 3, top). The intensity I(r,t_(o)) is             basically determined by the contrast material (with the             associated intensity I₀(t)) that was injected at the             injection time t_(i) and that has covered the distance L             between injection and observation point at the observation             time t_(o). Thus, the respective injection time t_(i) is             obtained by solving the equation

∫_(t_(i))^(t_(o))v(τ, r) τ = ∫_(t_(i))^(t_(o))v_(average) ⋅ κ(r) ⋅ p(τ) τ = L.

-   -   -    Here, κ(r) represents the varying velocity of the blood and             contrast agent over the vessel cross section. However, the             transport of contrast agent is not only influenced by             pulsatility but also by diffusion. The diffusion of contrast             agent is simulated by a Gaussian distribution curve G(γ,Δt)             where γ indicates the strength of the diffusion and Δt is             the duration of the diffusion (FIG. 3, element 301). Hence,             additional model parameters are required to model the             effects of pulsatile contrast agent transfer. These             parameters are γ, L, v_(Average) and κ(r). Preferably, the             degree of pulsatility Π is defined such that Π=0 indicates             stationary flow without pulsatile modifications and Π=1             indicates a complete stop of the flow in diastole. A degree             of pulsatility above 1 would then indicate backflow and             would be characterized by some values p(t) below 0.

    -   5. Optimizing the model parameters: The pulsatility model makes         a prediction of the time intensity curve at the observation         point. In an optimization procedure the prediction and the         observation are compared and the model parameters are adjusted         such that the error between the measured time intensity curve         and the model prediction is minimized, see FIG. 4.

    -   6. Displaying results: to the physician/interventionalist in an         appropriate way. Alternatively, results can be passed on to         algorithms that require the compensation of pulsatility.

Alternative first embodiments reduce the number of mechanisms that influence the transport of contrast agent due to pulsatility. As a consequence, the associated model parameters become superfluous and the prediction by the pulsatility model becomes more precise: In particular, the injection point and observation point are identical. Thus, only the mixing of blood and contrast agent has to be represented by the model. If the system is calibrated beforehand by imaging a contrast agent probe, and, if the injection pump provides the volumetric contrast agent flow, no model adaptation is required at all. In this latter alternative embodiment, an observation I₀(t) and all model parameters (c_(CA) and Q_(CA)(t)) except the pulsatile volumetric blood flow Q_(feed)(t) are known. Hence, a direct calculation of Q_(feed)(t) is possible (computed using equation (2) or step 4 of the above method).

The volume flow, degree of pulsatility, varying speed over a vessel cross section and diffusion parameters are adapted by a computer program to automatically determine these and provide them to a user.

Estimation of the pulsatility provides important information in itself. In addition, it supports reliable and reproducible blood flow assessment.

Referring now to FIG. 7, an apparatus 703 is illustrated for extracting pulsatility from an acquired sequence of images 712, comprising a processor 703 configured to receive an ECG signal 703 and the image sequence 702, and to control the extraction of pulsatility therefrom by several modules. The modules include a CA predictor module 704 for providing a predicted sequence, a template scaling/shifting module 709 that retrieves and transforms an appropriate template from a database of pre-defined templates 707 that covers all observable vascular subsystems in the human body and appropriately scales and shifts it, and a pulsatility extraction module 705 that compares the acquired sequence with the predicted sequence and derives a model in which the error therebetween is minimized. A model of a patient's pulsatility is output 710 that is based on the acquired sequence and ECG and is therefore only valid for the procedure performed.

A second preferred embodiment replays an acquired image sequence according to a heartbeat dependent time axis transformation. In doing so, the influence of the pulsatility on the displayed flow sequence is reduced. Its characteristic parameters are estimated by image processing methods from the sequences or selected by the end-user. The required information about the heart cycle is, for instance, extracted from the ECG Signal.

In an alternative second embodiment, pre- and post-treatment angiograms provide improved outcome control by the comparison of both adjusted replay sequences derived therefrom. To this end, both replay sequences are temporally adjusted such that the bolus starts from a given reference location (e.g. injection point) at the same time and the influence of pulsatility is removed using the time axis transformation. Subsequently, the sequences are replayed to the physician for comparison thereby.

In yet another alternative second embodiment, for an optimal image quality of the adjusted replay sequence at least one of a time varying frame rate or a high frame rate in combination with a varying tube current (allowing generation of the desired image frames after acquisition) is used for the acquisition of the angiograms. In doing so, the same or a similar time axis calibration as exploited in the replay of the blood flow sequence should be utilized for the variation of the frame rate or the tube current. The merits of this acquisition protocol are:

-   -   that the replayed frames are directly obtained in the x-ray         procedure or that at least a sufficient number of frames/high         dose angiograms are acquired during the essential phases of the         heart cycle, respectively.     -   that the dose exposure of the patient is reduced by avoiding         superfluous acquisitions or by minimizing the dose. In addition,         the warming-up of the x-ray tube is eased.

A method according to the second embodiment is illustrated in FIG. 5, in which a replay of adjusted pre- and post-treatment angiograms is indicated by the dashed lines marking the related processing steps. The method comprises the steps of:

-   -   1. Specifying a time axis transformation 502.5: The blood flow         is characterized by a change of the flow speed over the cardiac         cycle that is represented in a curve p(Ø_(ECG)) which is         transformed to a curve p(t) via observations of the patient's         cardiac activity, e.g. by ECG. This curve has to be         appropriately scaled, an average speed has to be added and a         phase shift or respective delay has to be applied such that a         modified template curve p(t) then represents the blood flow in a         given observation. This modification is a linear transformation         of the template curve such that in average, the volume flow Q is         obtained that flows with a degree of pulsatility Π. Preferably,         the degree of pulsatility Π is defined such that Π=0 indicates         stationary flow without pulsatile modifications and Π=1         indicates a complete stop of the flow in diastole. A degree of         pulsatility above 1 would then indicate backflow and would be         characterized by some values p(t) below 0.     -   2. Acquiring a sequence of images 502.6: In order to obtain a         constant frame rate for the replay after the time axis         transformation, a non-equidistant sampling of the time of the         real world is required. To be more precise, the sampling of the         real world axis is close, but equidistant during the systole and         wider during diastole. Since the time transformation affects the         time instances of acquisition, the time axis transformation         might also steer the acquisition. To this end, two different         possibilities exist:         -   a. The frame rate is adaptable. Hence, the images are             acquired only at or close to the predefined time instance.         -   b. A high frame rate in which the desired image frames are             generated after acquisition by binning/integrating.             Additionally, the frame rate interpolation becomes easier             since it ends up in a modified binning of the acquired             frames.     -   3. Determining the beginning of the systole and diastole 502.4:         The time axis transformation has to be conducted for each heart         cycle separately. To this end, the phase on the cardiac cycle         Ø_(ECG)(t) has to be determined first. One possibility to obtain         the required Ø_(ECG)(t) consists in exploiting an ECG signal.         Preferably, the phase is assigned by equal spacing between         subsequent R-peaks in this ECG. When this method is applied,         then the phase shift or delay of the ECG signal to the resulting         flow speed in the observation is one parameter that can be         manually adjusted or automatically determined such that the         predicted time-intensity curves best match the extracted ones.     -   4. Interpolating the frame according to time axis transformation         and replay 502.7: The time-axis transform t→t′ is now defined         according to

∫₀^(t^(′))p(τ) τ = t.

-   -    Due to the proposed time axis transformation the time instances         of acquisition are not equidistant any more, since in general         the frames have not been taken at exactly these required time         instances (possibly not even in case of the modified acquisition         protocols), a calculation of the intermediate frames is needed.         To this end, a frame rate interpolation can be utilized.         However, if a high frame rate in combination with a varying tube         current is used, the frame rate interpolation ends up in a         modified binning of the acquired frames. Subsequently after the         frame rate interpolation, the images are displayed with respect         to the new time scale t′.     -   5. Refining parameters 502.8: During inspection of the replay         with respect to time scale t′, the observer might notice         imperfections due to a non-optimally estimated degree of         pulsatility and phase shift of the template p(Ø_(ECG)). In         consequence, the possibility to modify these parameters by user         interaction can be provided by the replay system.     -   6. Controlling outcome 501: If, in addition a post treatment         angiogram is available, the described approach above can also be         utilized for an improved outcome control. To this end, the         following steps are required:         -   i. Determining time instances of replay 501.5: For a             comparison of the bolus propagation in both sequences the             bolus has to start from a reference location defined by the             position of the catheter in the vasculature (e.g. the             beginning of the aneurysm) at the same time. Since the             injection point and time and the average blood flow velocity             of both sequences is different, this demand does not apply             in general to the acquired/adjusted sequences. Hence, the             required time instances of replay for the post treatment             sequence have to be determined first, before a time             transformation can be conducted.         -   ii. Transforming Time Axis 501.3 through Replaying and             comparing adjusted time sequences 501.8—To achieve this,             both sequences are preferably processed according to the             processing described above for steps 1-5 but with individual             time-axis transformations t→t₁′ and t→t₂′. As a final degree             of freedom, in step 501.5, the temporal offset between the             sequences is adjusted such that the observed time-intensity             curves for one control point, preferably selected at the             feed of a pathology, have maximum overlap.

As an example of the application of the second embodiment and its alternative, the goal of many vascular interventions is to beneficially modify the blood flow, e.g. to increase flow through a stenosed section or to reduce the blood stream in an aneurysm. A comparison of diagnostic and post-interventional acquisitions is therefore desired to judge the impact of an intervention. A visualization of differences can give valuable information of the type “contrast agent now arrives faster at this segment” or “the amount of attenuation in this segment is always reduced”, which is available from a subtraction of pre- and post-interventional data.

To enable this subtraction, the influences of pulsatility and the moment of contrast injection have to be compensated for. For this compensation, a flow analysis of relevant sections of interest is required first, provided by using the first embodiment.

In the following example it is assumed that the average flow volumes Q1 and Q2 have been extracted together with their variation over time due to heartbeat and contrast injection in p₁(t) and p₂(t), respectively. Furthermore, TICs at the inflow into the relevant vascular substructure must be available. For comparable dynamic angiograms, an irregular re-sampling t→t₁′ and t→t₂′ is created such that:

∫₀^(t_(i)^(′))p_(i)(τ) τ = t

This temporal re-sampling converts both acquisitions into quasi-stationary acquisitions that only vary in the total amount of blood flow. The temporal offset between the two sequences is determined that results in the maximal correlation of the TICs read at the inflow into relevant vascular substructure. The comparison of two diagnostic angiograms is further enhanced when the same amount of contrast agent is applied over the same injection time. Preferably, the injection starts in the same phase over the cardiac cycle. After this temporal registration and regularization, differences can be visualized with grayscale overlays and colored overlays on a post-interventional sequence.

Referring now to FIG. 6 a-1 to 6 a-4, an aneurysm in a neurovascular artery is simulated with an intervention of stent insertion such that the aneurysm flow fraction is reduced in the post-interventional simulation. Frames from the post-interventional acquisition are shown in FIG. 6 a 1-4. After the temporal adjustment and registration described above, the subtraction images allow for comparison with a pre-interventional acquisition and reveal that contrast agent arrived earlier in the aneurysm before the intervention. This is coded as bright overlay in FIG. 6 b-1 to 6 b-4 and visualizes the effect of the intervention step, namely the slower flow of blood through the aneurysm.

A third embodiment provides a contrast agent injector for enabling reproducible and comparable flow acquisitions. The provided contrast agent injector measures ECG and always injects the contrast agent at a fixed point over the cardiac cycle, then at least the TICs at the injection site are similar, which removes one of the main unwanted variables in the acquisition of flow sequences.

In a third embodiment, the control of the CA injection is based on an ECG analysis. When the physician/interventionalist initiates the contrast agent injection, the injector does not start immediately, but rather starts automatically at a next appearance of a pre-defined cardiac phase in a live-observed ECG.

This third embodiment can be used for achieving:

-   -   outcome control based on comparison of CA dynamics pre- and post         intervention if a reproducible CA injection is provided.     -   different injector patterns can be adapted to the current heart         rate and assessment algorithms can request the phase in the         cardiac cycle when an observed bolus front arrives at a region         of interest in a vascular subsystem.     -   short contrast agent bursts, such that a flow dynamic at a         pre-selected phase in the cardiac cycle can be assessed.

An example of an apparatus 800 implementing the third embodiment is illustrated in FIG. 8, and provides a way to avoid the dependency of videodensitometric analyses on the time of injection. In a preferred third embodiment, control of the time of contrast agent injection is by a CA injection control module 802 and is based on ECG acquisitions 708. The QRS-complex or similar characteristics are detected on the acquired ECG signal 708 and, consequently, the heart rate and phase in the cardiac cycle is known, the latter through use of a phase determination module 803. With this information, the alternation of systolic and diastolic driving pressure in the vascular system is also known in advance. When activated by the physician/interventionalist, a CA control mechanism 804 (preferably an injection pump) does not start immediately, but the start is controlled by the control module 802 to occur at the next arrival of a pre-selected phase of the cardiac cycle, as determined by the phase determination module 803 from the simultaneously acquired ECG signal 708. This third embodiment requires a reproduced position of the injection point 713, which is preferably provided by at least one of imaging of the catheter itself and observation of the length to which the catheter is advanced.

An analysis system embodiment 900 comprises an integrated workstation 901 for quantitative or comparative analysis of CA dynamics and includes an integrated architecture of selected first through third embodiments and their alternatives of the present invention. For example, the workstation illustrated in FIG. 9 includes an ECG acquisition system 712, a CA injection control system 800 to provide start and stop commands for the CA injector and a dedicated image analysis subsystem 910 that adjusts to the current acquisition 712 and vice versa. This workstation 901 is further configured to update the estimation of the current phase in the cardiac cycle from live ECG acquisitions 712. Controls are included in a CA injection control system 800 that is interfaced to the workstation 901 so that the workstation can start and stop a contrast injection pump which synchronizes to the estimated current cardiac phase according to requirements pre-determined for specific analysis tasks.

Applications of the analysis embodiment include:

-   -   The introduction of patterns in the contrast agent injection         Q_(CA)(t) with an increased frequency in comparison to the heart         beat can provide features of improved signal quality to analysis         algorithms. The parameters of such an injection pattern can be         controlled based on ECG and related information (e.g. heart         rate) to allow for an improved adjustment of analysis algorithms         and their respective injection patterns.     -   Short injector bursts at a defined phase can control the arrival         of the CA bolus in a region of interest at pre-selected phases         in the cardiac cycle. The systematic adjustment of the injection         of such a small burst can give quantitative information         regarding the blood flow dynamics at a region of interest over         the full heart cycle. In a dedicated protocol, this short burst         could be followed by a longer CA injection so that the vascular         system and the CA dynamics are still visible to a human         observer.

While the preferred embodiments of the present invention have been illustrated and described, it will be understood by those skilled in the art that the system, apparatus and methods as described herein are illustrative and various changes and modifications may be made and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. In addition, many modifications may be made to adapt the teachings of the present invention to a particular situation without departing from its central scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the present invention, but that the present invention include all embodiments falling within the scope of the claims appended hereto as well as all implementation techniques and not just the various implementation architectures used herein as examples. 

1. A method for determining a degree of pulsatility from a sequence of images, comprising the steps of: acquiring (602.6) with an imaging system a time intensity sequence of images (702) of the concentration of a known contrast agent resulting from transport of a mixture of blood and the known contrast agent through a vascular system of a patient; providing a contrast agent prediction model (707) that is based on a function of the mixing of the known contrast agent with an observed time-dependent blood flow; and extracting a degree of pulsatility from the acquired sequence of images (702) using the provided contrast agent prediction model.
 2. The method of claim 1, wherein the imaging system is selected from the group consisting of interventional x-ray, ultrasound, and repeated acquisitions using angiography with computed tomography or magnetic resonance imaging.
 3. The method of claim 2, wherein: the acquiring step further comprises the steps of: a. varying (711) a concentration of the contrast agent over time at an injection site thereof, b. imaging (701) the known contrast agent at the injection site (713) to obtain an observed sequence of a concentration thereof, and c. obtaining a predicted sequence (704) of the concentration of the known contrast agent at the injection site (713) by use of the contrast agent prediction model; and the extracting step further comprises the step of matching (706) the observed sequence with the predicted sequence.
 4. The method of claim 3, wherein the imaging step (701) further comprises the step of imaging the known contrast agent downstream of the injection site at a region of interest (710) to obtain an observed sequence (702) thereof, wherein the function on which the contrast agent prediction model is based additionally takes into account the transport of the mix of contrast agent and blood downstream of the injection site by each of convection, dispersion due to varying speeds over a vessel cross section, diffusion and the variable transport speed due to the pulsatility itself.
 5. The method of claim 4, wherein the acquiring step further comprises the step of simultaneously obtaining an electrocardiogram (ECG) (712) signal (708) from the patient; and further comprising the steps of: a. identifying a phase Ø_(ECG) of a cardiac cycle of the patient from the ECG (708), and b. determining the time-dependent blood flow over the cardiac cycle with a template for a pulsatility p(Ø_(ECG)) having flow speeds as a function of the identified phase Ø_(ECG).
 6. The method of claim 5, wherein the matching step (706) further comprises the step of obtaining an optimal match by performing the steps of: scaling (709) the template; adding the scaled template to an offset of stationary flow to obtain a modified stationary flow; and shifting (709) the modified stationary flow over the cardiac cycle.
 7. The method of claim 5, wherein the template is a simplified template comprising a high average flow speed in systole and a low average flow speed in diastole.
 8. The method of claim 7, wherein the matching step further comprises the step of obtaining an optimal match by performing the steps of: scaling (709) the template; adding the scaled template to an offset of stationary flow to obtain a modified stationary flow; and shifting (709) the modified stationary flow over the cardiac cycle.
 9. A method for visualization (904) and analysis (910) of an acquired sequence of images (702) of a blood flow of the advance of a contrast agent through a region of interest (710) in a vascular system of a patient, comprising the steps of: obtaining (703) information on the pulsatile nature of the blood flow (710); modifying the sequence of images using the obtained information to remove therefrom an influence of pulsatile flow (902); and providing both the acquired sequence of images and the modified sequence of images to a blood flow visualization and analysis system (904).
 10. The method of claim 9, wherein where the information on the pulsatile nature of the blood flow (710) is obtained by one of the techniques selected from the group consisting of manual interaction and the method of claim
 6. 11. The method of claim 9, wherein where the information on the pulsatile nature of the blood flow (710) is obtained by one of the techniques selected from the group consisting of manual interaction and the method of claim
 8. 12. The method of claim 11, wherein the acquired sequence comprises (702) two sequences including a sequence acquired before an interventional step and a sequence acquired after an interventional step.
 13. The method of claim 12, wherein the visualizing (904) and analyzing step comprises the step of visualizing differences (911) between the two sequences.
 14. The method of claim 13, wherein the step of visualizing (904) differences further comprises the step of using a color overlay (911) for one of the two sequences.
 15. The method of claim 14, further comprising the step of determining a temporal offset (903) between the two sequences such that a concentration of contrast agent at a pre-selected point in time has maximal similarity between the two sequences.
 16. A method for controlling contrast agent injection (803) into a region of interest (710) in a vascular system of a patient, comprising the steps of: determining (802) a phase in a cardiac cycle of the patient; and injecting at an injection point (713) the contrast agent into the region of interest (701) of the vascular system based on the determined phase.
 17. The method of claim 16, wherein the determining step further comprises the steps of: obtaining an electrocardiogram signal (708) from the patient in parallel with the injecting step; and determining the phase the cardiac cycle of the patient from the ECG signal (708).
 18. The method of claim 17, wherein: said regions of interest (710) is downstream from the injection point; and the determining step further comprises the step of determining the phase in the cardiac cycle when the injected contrast agent arrives at the region of interest (710).
 19. The method of claim 18, further comprising the steps of: systematically varying a time of injection with the cardiac cycle; and giving short contrast agent injections at the systematically varied times of injection.
 20. The method of claim 19, wherein the systematically varying step is at a high-frequency such that a high-frequency pattern of contrast agent injection is achieved.
 21. The method of claim 20, where the pattern is synchronized with a heart beat of the patient.
 22. An apparatus (800) for the control of a contrast agent injection into a vascular system of a patient, comprising: a contrast agent injection control module (803) to control a means for contrast agent injection; and a processor (801) to receive an electrocardiogram (ECG) signal (708) and direct the contrast agent injection control module (803) to turn on and turn off the means for contrast agent injection, using on a pre-determined method based on the ECG signal (708).
 23. The apparatus (800) of claim 22, wherein the pre-determined method is the method of claim
 21. 24. The apparatus of claim 23, wherein the means for contrast agent injection is an injection pump (804).
 25. The apparatus (800) of claim 22, wherein: the apparatus further comprises a phase determination module (802) to determine the phase of a heart cycle of the ECG signal (708); and the pre-determined function is a function of the determined phase of the heart cycle.
 26. The apparatus (800) of claim 25, wherein the means for contrast agent injection is an injection pump (804).
 27. An apparatus (700) for extraction of a degree of pulsatility from an image sequence (702) of the transport of contrast agent in a vascular system of a patient, comprising: a pulsatility template database (707) including pre-determined templates as a function of a cardiac phase for observable vascular systems; and a processor (703) to receive an electrocardiogram (ECG) signal (708) and the image sequence and use the database (707) to perform the method of claim 8 to extract the degree of pulsatility from the image sequence.
 28. A system (900) for visualization and analysis of an original sequence of images (702) of blood flow of the advance of a contrast agent through a region of interest in a vascular system of a patient, comprising: an electrocardiogram (ECG) acquisition system (712) to acquire an output an ECG signal (708) of a heart cycle of the patient; a contrast injection control system (800) connected to the ECG system and configured in accordance with claim 26; a pulsatility extraction system (703) connected to the ECG system and configured in accordance with claim 27; and a visualization and analysis subsystem (900) connected to the ECG system and including: an image pulsatility removal module (902) to remove an effect of pulsatility from the original sequence of images to produce a second sequence of images, a temporal offset determination module (903) to offset the second from the original sequence of images, a visualization module (904) to simultaneously visualize the original (702) and offset sequence of images on a display of an image analysis system (910), and an image analysis subsystem (910) to control the injection of contrast agent by the contrast injection control system (800) prior to a subsequent acquisition of at least one further sequence of images for visualization and analysis by the subsystem (900).
 29. A system (900) for visualization and analysis of an original sequence of images (702) of blood flow of the advance of a contrast agent through a region of interest in a vascular system of a patient, comprising: an electrocardiogram (ECG) acquisition system to acquire an output an ECG signal (708) of a heart cycle of the patient; a contrast injection control system (800) connected to the ECG system and configured in accordance with claim 26; a pulsatility extraction system (703) connected to the ECG system and configured in accordance with claim 27; and a visualization and analysis subsystem (900) connected to the ECG system configured to perform the method of claim
 9. 